1. Field of the Invention
This invention relates to the production of thin film resistors. More particularly, this invention relates to thin film resistors made using special formulations of chromium, silicon, and carbon.
2. Description of the Prior Art
Thin film resistors are useful in integrated circuit structures where high sheet resistance is required. While doped polysilicon materials are conventionally used in digital circuitry, analog circuits require more precision in the resistance values including low temperature coefficients of resistance (TCR) and high stability over lifetime. A number of materials, including alloys such as nickel-chromium, have been previously used. A paper by Robert K. Waits entitled "Silicide Resistors for Integrated Circuits", published in the Proceedings of the IEEE at volume 59, No. 10 (October, 1971) at pages 1425-1429, lists a number of thin film resistor materials including a number of metal silicides, including molybdenum silicide and chromium silicide.
While the use of silicide materials for producing thin film resistors has been preferred over other materials, silicide materials are also not without problems. The same author, Robert K. Waits, describes low temperature failures of unpassivated thin film silicide resistors in "Silicon-Chromium Thin-Film Resistor Reliability" published in Thin Solid Films, volume 16 (1973) at pages 237-247.
It has been found that a material to be used in the production of thin film resistors should, ideally, possess a number of characteristics. First, the material should have a resistivity of greater than about 800 to less than about 1200 ohms per square, not only to provide a sufficiently resistive material, but to permit application, to a substrate, of a resistor film of reasonable thickness, e.g., about 100-200 Angstroms, to insure uniformity or reproducibility of the film resistivity despite slight processing differences in film thickness. The uniformity of the resistivity of the film should provide a variation in resistance at various portions of the film of not greater than about 14%.
The temperature coefficient of resistance (TCR) of such a material should be low, i.e., less than about 200 ppm per degree Centigrade over the operating temperature range, i.e., -25.degree. to +125.degree. C.
The resistance of the material should not substantially change during subsequent processing of the integrated circuit structure after annealing of the film, e.g., subsequent exposure to elevated temperatures under the annealing temperature. The term "substantial change", as used herein to describe changes in resistivity due to processing, is intended to define a change in resistance of not more than 0.1%.
The annealing temperature of such a resistor material should not exceed about 500.degree. C. to avoid encountering problems with any aluminum films in the integrated circuit structure. Therefore, the resistor material must be annealable at temperatures of 500.degree. C. or less.
The resistor material must be easily applicable to the substrate in an accurate manner since substantial variations in thickness will result in variations in the resistivity. If the material is to be applied, for example, by sputtering, the material must be responsive to reasonable gas pressures and target voltages, i.e., a pressure equal to or less than less than 2.0.times.10.sup.-7 Torr and a voltage of from about 1000 to 1400 volts, preferably 1200 volts, to provide a film of uniform thickness.
Since the resistor material can be effected by the substrate, including not only the flatness of the substrate, but the mechanical stability as well, the resistor material should possess a temperature coefficient of expansion matching that of thermally grown or chemical vapor deposited (CVD) silicon oxide, including phosphorus doped oxides since these will be the normal substrate materials under the resistor film.
Finally, the resistance of the film must be stable with age. An acceptable absolute lifetime stability will result in an absolute shift of less than a 0.1% shift of the resistance over the lifetime of the structure, e.g., over a 2000 hour period at 150.degree. C. The resistor film should also have a good matching shift stability over a lifetime as well, i.e., the degree of variation present in a resistor array. The matching shift should also be less than 0.1% over a 2000 hour period at 150.degree. C.